I've been using a D100 since last July and I have made quite a few prints on 13 x 19 paper (approximately 12 x 18 prints usually). Before printing at that size, I increase the file resolution to at least 240ppi. Although I have Genuine Fractals, I have been generally satisfied using successive iterations of Photoshop bicubic interpolation, similar to the "stairstep" method.
Overall, I am delighted with the results. I very seldom use my film scanner anymore. The D100 generally produces better results, with a lot less work. The accurate colors, absence of noise, and shadow detail are all great.
There have been endless arguments on the web about how much digital resolution is necessary to match 35mm. I won't try to re-hash those (or re-ignite the flames). Instead I'll just leave it at the fact that I find I am more frequently satisfied with prints at the size you mention using the D100 than with scanning 35mm film on a desktop film scanner. That is not the same thing as saying that there aren't times I wish for more resolution--some scenes just benefit from a lot of detail and you notice the lack of resolution as you enlarge beyond 8 x 10 (but that is true with 35mm film). Print quality isn't just resolution; it is resolution, contrast, color, absence of noise, dynamic range, ability to hold detail in the shadows and highlights, etc. When you look at all those factors, I think you'll be happy with the D100 at the print sizes you mention.
By the way, there is a partial solution to the resolution question for landscape work: stitching several D100 frames together digitally. I just recently returned from a trip to Death Valley. Most of my shots were conventional single frame shots, but I also did a few panoramas. Right now I am looking at a 12 x 42 print made from 8 D100 frames (taken in portrait orientation) and the detail is unbelievable!